You can find required information concerning personalization in the “Oracle? Application Framework Personalization Guide” (for R12.1 it is Part No. E12646-02).
In the Guide, there is an Example of how to add a Text Message to a Page thru Personalization – check page 4-25 “Example: Adding New Instruction Text to a Page”.

Form Personalization is accessible thru “Help” Menu (Help > Diagnostics > Custom Code > Personalize).
In case “Diagnostics” Menu is not available to you, then you should set System Profile Option “Hide Diagnostics menu entry” for your User to “No”.
You can follow Document from Form Personalizations in Oracle E-Business Suite (Release 12) [ID 395117.1] (starting from page 6 to get the instructions for Message Type).